The graph's two axes represent the membrane potential (mV) and the time (ms). We can see four phases: the resting potential, repolarisation, depolarisation and then rest potential again. It all starts with a stimulus that excites the neuron. This electric stimulation must reach a threshold (-50 mV) because otherwise no action potential will take place as it is an all-or-none event! In resting potential, the potassium channels are open and K+ flows freely based on the concentration and electrical gradient. At -50 mV, K+ channels close and voltage-gated sodium channels open, so Na+ flows into the cell. The cell therefore becomes more positive on the inside (depolarises) and soon reaches +40 mV, where Na+ channels shut and K+ channels open and the cell repolarises, hyperpolarises even. However, there is an ion imbalance which prevents the next action potential from taking place. This imbalalnces is reversed by the sodium-potassium pump, which pushes Na+ out of the cell and draws K+ inside. As we reached the original state, the resting potential, another action potential can finally happen.